There are predominantly two types of paper produced, lignin-free paper and lignin-rich paper. Lignin-rich paper is inferior to lignin-free paper in that it is susceptible to photo- and thermal-degradation. When lignin-rich paper is exposed to light, for example sun-light or incandescent-light, the optical properties of the paper degrade rapidly. The most obvious effect is that the paper becomes "yellow" and is aesthetically unpleasing. For a review of photo-yellowing the reader is directed to: Leary, G. L. Recent Progress in Understanding and Inhibiting the Light-Induced Yellowing of Mechanical Pulps, J. Pulp Paper Sci., Vol. 20(4), J154-J160 (1994), the disclosure of which is incorporated herein by reference.
The lignin present in pulp has a complex structure containing many functionalities. The structure of lignin varies depending upon the species of tree from which the pulp is obtained and also upon the environmental conditions under which the tree is grown. However, irrespective of the species of tree or conditions under which the tree is grown, lignin contains carbonyl functionalities and phenolic groups.
Lignin-rich paper, due to its inferior aging properties, is used for short-term usage, e.g., newspapers and telephone directories. Generally newsprint has a lignin content of about 25-35%. The advantages of lignin-rich paper are its cheapness, high-yield and ease of production. An example of lignin-rich paper is paper manufactured from thermomechanical pulp (TMP). Lignin-free paper is produced from chemically treated pulp. An example of lignin-free paper is that produced by the KRAFT process which involves the heating of pulp with a strongly basic aqueous solution.
A method of extending the lifetime (i.e., inhibiting the yellowing) of lignin-rich paper such as TMP paper would be of great value, as it would permit the use of lignin-rich paper in areas normally reserved for the more expensive lignin-free paper, e.g., in mid-term applications, such as forms which require a life extending from a few months to a few years. It is to this effect that the present invention is aimed.
The aforementioned article by Leary discusses many strategies that have been tried for inhibiting the light-induced yellowing of lignin-containing pulp and paper. Examples include:
i) reductive processes. Sodium borohydride treatment of lignin-rich paper, to transform carbonyl groups into the corresponding alcohols or hydroquinones, has been tried but without success. Reductive processes also have a detrimental effect on the mechanical properties of the paper produced; PA1 ii) adding radical scavengers. This gives temporary protection to high-yield pulps but the scavengers are eventually consumed and yellowing resumes; PA1 iii) acetylating or methylating lignin phenolic groups to prevent phenoxy radical formation. Acetylation can substantially retard yellowing. However, acetylation adversely affects the physical properties of the paper and it is not easy to acetylate lignin without also acetylating the cellulose present in the paper, which is undesirable because it reduces the hydrogen bonding that holds the fibers together in the paper.
An understanding of paper making is an advantage in understanding the present invention and where best to apply the present invention. A typical modern paper machine begins with a flow spreader or distributor, conveying a dilute aqueous fiber suspension (0.1-1% fibers) to a headbox which delivers a jet of the suspension or slurry through a sluice across the full width of the machine, which may be almost 10 meters in width in some large machines. In the headbox, the fibers are dispersed and the flow rate adjusted as well as possible so that the jet is delivered onto a moving endless fine-mesh wire screen with uniform composition, flow rate and velocity, to generate a travelling paper web. The pressure in the headbox and its sluice opening are adjusted so that the jet velocity matches the speed of the wire screen, which may be up to 1220 meters.minute.sup.-1 for newsprint. The proper stock flow per unit width corresponds to the desired weight of pulp per unit area.
A large proportion of water is removed from the paper web by a series dewatering steps, for example using rolls, foils and vacuum boxes. The formed paper sheet after these dewatering steps contains about four to five parts water to one part solids and is subjected to a felt press, which is a fine-textured, usually synthetic, fabric. The wet paper web that is formed is then passed with one or more press felts through two or more press-roll nips, where water is squeezed out and the paper mat is compacted. After the paper web has been subjected to mechanical removal of water most of the remaining water, down to 5-10%, must be removed by evaporative processes. These evaporative processes may include, for example, passing the paper through heated rollers, use of ventilating devices which blow air of a controlled temperature and humidity, or use of radiant heating, dielectric or microwave heating.
It is an object of the invention to provide a process that inhibits the yellowing of lignin-containing paper, which process is acceptable for use in the pulp and paper industry. Hence, the process should be a) cheap b) facile c) non-toxic in the plant d) non-toxic in the product and e) not result in paper with inadequate mechanical properties.